Create Public Initializer for DebugVisitor

I updated DebugVisitor. I added a public initializer to allow
DebugVisitor to be used by applications that consume the Down framework
for generating debug output from Markdown ASTs.

Source/AST/Visitors/DebugVisitor.swift

@@ -18,6 +18,8 @@ public class DebugVisitor {
     private var indent: String {
         return String(repeating: "    ", count: depth)
     }
+
+    public init() {}
 
     /// Debug representation of node.
     private func report(_ node: Node) -> String {